MicroLogix PLC

The human brain is an incredible machine. Over the past few years, I’ve learned some amazing things. Did you know that a woman’s brain when pregnant with her first child will shrink, reconfigure itself, and then expand with the whole new configuration that’s needed for Motherhood? Wow! Did you know that cab drivers in London who had to memorize (before GPS) every little street in London had these huge neural networks in their head and developed an amazing cognitive capacity?

I think that the automation specialists you find at Allen–Bradley distributors must have the same kind of amazing brain structures. To understand the entire line of PLCs from the Nano and Micro controllers all the way up to the large control systems with ControlLogix and CompactLogix is beyond my capabilities. Those lines have something for everybody. If you have a control problem, I don’t doubt that you can find a solution in the Allen–Bradley PLC line.

What really geometrically expands that understanding challenge is all the communications options for all those lines. You have all the legacy communications like DF/1, the open protocols like Modbus and DNP3, the I/O protocols like DeviceNet and Profibus DP, and, of course, the Ethernet networks like Modbus TCP, EtherNet/IP, and Profinet IO. How the automation specialists deal with all this complexity is beyond me.
This week I was working with a customer to understand just one PLC, the MicroLogix 1400. The 1400 is a higher end controller in the MicroLogix line with more I/O, enhanced counters, pulse outputs, and a very cool LCD panel. And you can expand it by adding I/O modules to it, and that makes it pretty flexible for machine builders and other applications.

On the communications side, network connectivity is improved over the earlier MicroLogix models. It has the standard serial ports and can do the legacy protocols: DF1, DH-485, Modbus RTU, DNP3, and straight ASCII, but I would never recommend anyone to use the ASCII capabilities of any PLC product. A much better option is to use one of our ASCII gateways: the serila ASCII to PLC gateway (435NBX product line) or the Ethernet TCP/IP ASCII to PLC gateway (490NBX product line). These products provide a much better user interface, have more customization capabilities, and have been proven leaders in moving barcode, RFID, and other ASCII data to PLCs for going on ten years now.

As a component of a sensor network, the 1400 leaves something to be desired. The MicroLogix 1400 doesn’t support any of the sensor protocols, specifically Profibus DP or DeviceNet. You need a gateway to add those features.

When we look at Ethernet and EtherNet/IP communications, the operation of the Micrologix controllers and specifically the 1400 gets messy. It’s somewhat complicated, but there are two key points to understand:

• EtherNet/IP has come to mean in general conversation a cyclic I/O connection. A scanner device sends outputs to an Adapter device and the Adapter device sends inputs to a Scanner device. This usually happens every 10 msec, but sometimes the inputs move at one rate and the outputs move at another rate.

• In Rockwell literature, EtherNet/IP has a broader meaning. EtherNet/IP means that a device supports a CIP (Common Industrial Protocol) connection over EtherNet/IP. A device that can send and receive explicit messages (the non-cyclic messages) is listed as having an EtherNet/IP connection. That’s not untrue, but is confusing to people who have only a general understanding of the technology.

In that mode, the MicroLogix 1400 can read or write the Assembly Object of an EtherNet/IP Adapter device, so it can function somewhat like a ControlLogix Scanner would, but it can only do that intermittently using a message instruction. It can read and write the Assembly object of an Adapter which is where the I/O data is located. So, it can move Inputs from the Adapter to the MicroLogix 1400 and send outputs from the MicroLogix 1400 to an EtherNet/IP Adapter. That’s unlike the ControlLogix EtherNet/IP Scanner which cyclically sends those inputs and outputs at a much faster rate.

To use the MicroLogix 1400 as an EtherNet/IP Scanner device, you need to use the RTA MicroLogix EtherNet/IP Scanner gateway. This device acts as an EtherNet/IP Scanner and lets the MiccroLogix1400 perform the same functions as a ControlLogix or CompactLogix PLC. Your data table in the MicroLogix 1400 is connected to the I/O data of the EtherNet/IP Adapters just like it would be for a ControlLogix controller.

The MicroLogix PLC product line makes choosing a communication option very difficult. Not as difficult as having a baby or memorizing the London Street map, but difficult. At RTA we may not have the neural networks of a London cab driver, but we do know MicroLogix and MicroLogix 1400 capabilities and can help you integrate those PLCs into your architectures.

John S Rinaldi

John Rinaldi is Chief Strategist, Business Development Manager and CEO of Real Time Automation (RTA). After escaping from Marquette University with a degree in Electrical Engineering, John worked in various jobs in the Automation Industry before once again fleeing back into the comfortable halls of academia. At the University of Connecticut, he once again talked his way into a degree, this time in Computer Science (MS CS). John is a recognized expert in industrial networks and the author of Four books: The Industrial Ethernet Book, OPC UA: The Basics, Modbus: The Everyman’s Guide to Modbus, and his latest, OPC UA - Unified Architecture: The Everyman's Guide to OPC UA.